1. Field of the Invention
The present invention relates to a driving circuit that drives light emitting diodes, and in particular to a light-emitting diode driving apparatus that drives light emitting diodes by using AC power supply.
2. Description of the Related Art
In recent years, significant attention is given to light emitting diodes (hereinafter, occasionally referred to as “LEDs”) as lighting sources. The reason is that LEDs can be driven at low power consumption as compared with filament lamps or fluorescent lamps. LEDs are small, and have shock resistance. In addition, LEDs are less prone to blow out. Thus, LEDs have these advantages.
In the case of lighting sources, it is desirable that commercial AC power for home use is used as power supply for lighting sources. However, LEDs are devices driven by DC power. LEDs emit light only when applied with a current in the forward direction. Also, in the case of LEDs that are currently typically used for lighting use, the LEDs operate on DC power at a forward directional voltage Vf of about 3.5 V. LEDs do not emit light if a voltage applied to the LEDs does not reach Vf. On the other hand, after a voltage applied to the LEDs exceeds Vf, an excessive amount of current may flow through the LEDs. Accordingly, it can be said that DC power is suitable for driving LEDs.
To satisfy the contradictory conditions, various types of LED driving circuits have been proposed which use AC power. For example, a method has been proposed which switches LEDs so that a Vf total value is changed in accordance with a varying voltage value (see Japanese Patent Laid-Open Publication No. JP 2006-147,933 A). In this method, a number of LEDs connected to each other in series are assigned to blocks 161, 162, 163, 164, 165 and 166 as shown in a circuit diagram of FIG. 14. The LED blocks 161 to 166 are selectively connected to the power supply in accordance with the voltage value of input voltage of rectified waveform by a switch control portion 167 consisting of a microcomputer so that a Vf total value is changed in a stepped manner. As a result, as shown by a voltage waveform in a timing chart of FIG. 15, since the LEDs can be driven by a plurality of rectangular waves corresponding to the rectified waveform, the LED operation efficiency can be improved as compared with the ON-duty in the case of only single rectangular wave.
On the other hand, the applicant has been developed an AC multi-stage circuit which includes a plurality of serially-connected LED blocks operated by an AC current after full-wave rectification, each of the plurality of LED blocks having a plurality of serially-connected LEDs (Japanese Patent Laid-Open Publication No. JP 2011-40,701 A). As shown in FIG. 16, this AC multi-stage circuit 1600 subjects a current from an AC power supply AP to full-wave rectification in a bridge circuit 1602 so that the LED blocks of multi stages are supplied with the current after the full-wave rectification. As the LED blocks of multi stages, first, second and third LED blocks 1611, 1612 and 1613 are serially connected to each other. A first LED current control transistor 1621A is turned ON/OFF to connect/disconnect a first bypass BP1601, which bypasses the second LED block 1612, based on the current amount in the first LED block 1611. A second LED current control transistor 1622A is turned ON/OFF to connect/disconnect a second bypass BP1602, which bypasses the third LED block 1613, based on the current amount in the first and second LED blocks 1611 and 1612. When the third LED current control transistor 1623A is turned from ON to OFF, a current cannot flow through a third bypass path BP1603, which bypasses an LED current restriction resistor 1603A. As a result, a current starts flowing through the LED current restriction resistor 1603A. The AC multi-stage circuit 1600 can keep power supply efficiency high, and additionally improve the use efficiency and the power factor of LEDs.
This light-emitting diode driving apparatus includes first, second and third current detection transistors 1631A, 1632A, and 1633A that are used to control ON/OFF of the first, second and third LED current control transistor 1621A, 1622A, and 1623A, respectively. Accordingly, the parts count will increase, and the circuit construction will be complicated.
On the other hand, since the first, second and third current detection transistors 1631A, 1632A, and 1633A are independently activated, it is necessary to precisely adjust the activation points of the first, second and third current detection transistors 1631A, 1632A, and 1633A to switch the first, second and third current detection transistors 1631A, 1632A, and 1633A at proper timing. In particular, noise and the like may cause activation timing point variation. For this reason, it is not easy to design the circuit with high reliability.
The present invention is devised to solve the above problems. It is a main object of the present invention to provide a light-emitting diode driving apparatus that can switches driving circuit activation at proper timing by using a simple circuit.